Methods of using fet labeled oligonucleotides that include a 3&#39;-5&#39; exonuclease resistant quencher domain and compositions for practicing the same

ABSTRACT

Methods and compositions are provided for detecting a primer extension product in a reaction mixture. In the subject methods, a primer extension reaction is conducted in the presence of a polymerase having 3′→5′ exonuclease activity and at least one FET labeled oligonucleotide probe that includes a 3′→5′ exonuclease resistant quencher domain. Also provided are systems and kits for practicing the subject methods. The subject invention finds use in a variety of different applications, and are particularly suited for use in high fidelity PCR based reactions, including SNP detection applications, allelic variation detection applications, and the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/222,943, filed Aug. 15, 2002, which is continuation-in-part of U.S.application Ser. No. 10/087,229, filed Feb. 27, 2002, the disclosures ofwhich are herein incorporated by reference.

INTRODUCTION

1. Technical Field

The technical field of this invention is the polymerase chain reaction(PCR); and particularly high fidelity real-time PCR.

2. Background of the Invention

The polymerase chain reaction (PCR) is a powerful method for the rapidand exponential amplification of target nucleic acid sequences. PCR hasfacilitated the development of gene characterization and molecularcloning technologies including the direct sequencing of PCR amplifiedDNA, the determination of allelic variation, and the detection ofinfectious and genetic disease disorders. PCR is performed by repeatedcycles of heat denaturation of a DNA template containing the targetsequence, annealing of opposing primers to the complementary DNAstrands, and extension of the annealed primers with a DNA polymerase.Multiple PCR cycles result in the exponential amplification of thenucleotide sequence delineated by the flanking amplification primers.

An important modification of the original PCR technique was thesubstitution of Thermus aquaticus (Tag) DNA polymerase in place of theKlenow fragment of E. coli DNA pol I (Saiki, et al. Science,230:1350-1354 (1988)). The incorporation of a thermostable DNApolymerase into the PCR protocol obviates the need for repeated enzymeadditions and permits elevated annealing and primer extensiontemperatures which enhance the specificity of primer:templateassociations. Taq DNA polymerase thus serves to increase the specificityand simplicity of PCR.

Although Taq DNA polymerase is used in the vast majority of PCRperformed today, it has a fundamental drawback: purified Taq DNApolymerase enzyme is devoid of 3′ to 5′ exonuclease activity and thuscannot excise misinserted nucleotides (Tindall, et al., Biochemistry,29:5226-5231 (1990)). Consistent with these findings, the observed errorrate (mutations per nucleotide per cycle) of Taq polymerase isrelatively high; estimates range from 2×10⁻⁴ during PCR (Saiki et al.,Science, 239:487-491 (1988); Keohavaong et al. Proc. Natl. Acad. Sci.USA, 86:9253-9257 (1989)) to 2×10⁻⁵ for base substitution errorsproduced during a single round of DNA synthesis of the lacZ gene (Eckertet al., Nucl. Acids Res. 18:3739-3744 (1990)).

Polymerase induced mutations incurred during PCR increase arithmeticallyas a function of cycle number. For example, if an average of twomutations occur during one cycle of amplification, 20 mutations willoccur after 10 cycles and 40 will occur after 20 cycles. Each mutant andwild type template DNA molecule will be amplified exponentially duringPCR and thus a large percentage of the resulting amplification productswill contain mutations. Mutations introduced by Taq DNA polymeraseduring DNA amplification have hindered PCR applications that requirehigh fidelity DNA synthesis. Several independent studies suggest that 3′to 5′ exonuclease-dependent proofreading enhances the fidelity of DNAsynthesis (Reyland et al, J. Biol. Chem., 263:6518-6524, 1988; Kunkel etal, J. Biol. Chem., 261:13610-13616, 1986; Bernad et al, Cell,58:219-228, 1989). As such, it is desirable, where possible, to includea 3′ to 5′ exonuclease-dependent proofreading activity in PCR basedreactions. For example, If Taq DNA Polymerase (error rate 2×10⁻⁴) isused to amplify a 100 by sequence for 40 cycles by PCR, about 55% of theamplification products will contain one or more errors. In contrast, ifa Pwo DNA Polymerase having proof-reading activities is used for theamplification, only 10% of the products will contain an error under thesame conditions. The error rate produced by a mixture of Taq DNAPolymerase and a proofreading DNA Polymerase between these two values(Cline et al, Nucleic Acids Res., 24(18):3546-51, 1996).

In many PCR based reactions, a signal producing system is employed,e.g., to detect the production of amplified product. One type of signalproducing system that is attractive for use in PCR based reactions isthe fluorescence energy transfer (FET) system, in which a nucleic aciddetector includes fluorescence donor and acceptor groups. FET labelsystems include a number of advantages over other labeling systems,including the ability to perform homogeneous assays in which aseparation step of bound vs. unbound labeled nucleic acid detector isnot required.

In such real time detection systems using a FET labeled nucleic aciddetector, high fidelity amplification is critical. Any error insequences where a FET labeled nucleic acid detector binds can causeprobes not to bind or wrong probes to bind in the case of allelediscrimination, resulting in weak signal or the wrong signal beingproduced. For example, if a 30 by PCR fragment which is the target of aFET labeled probe is amplified using Taq DNA Polymerase for 40 cycles,about 22% of the amplification fragments will contain one or moreerrors. In contrast, if a Pwo DNA Polymerase having proof-readingactivities is used for the amplification, only 3% of the amplificationfragments will contain an error under the same conditions. Therefore,the standard low fidelity amplification can cause a decrease insensitivity or mis-typing in the case of allele discrimination.

However, as discovered by the current invention a disadvantage ofcurrently available FET labeled nucleic acids having TAMRA or Dabcyl asa quencher is that such nucleic acids are subject to 3′→5′ exonucleasedegradation. Accordingly, such FET labeled nucleic acids are notsuitable for use in high fidelity PCR applications, where 3′→5′exonuclease activity, i.e., proofreading activity, is present.

As such, there is significant interest in the identification anddevelopment of FET labeled nucleic acids that can be used in highfidelity PCR applications.

Relevant Literature

U.S. patents of interest include: U.S. Pat. Nos. 5,538,848 and6,248,526. Also of interest are: WO 01/86001 and WO 01/42505.

SUMMARY OF THE INVENTION

Methods and compositions are provided for detecting a primer extensionproduct in a reaction mixture. In certain embodiments of the subjectmethods, a primer extension reaction is conducted in the presence of apolymerase having 3′′5′ exonuclease activity and at least one FETlabeled oligonucleotide probe that includes a 3′→5′ exonucleaseresistant quencher domain. In certain embodiments a nucleic acidintercalator is covalently bonded to the FET labeled oligonucleotide. Incertain embodiments, a minor groove binder is employed in the reaction.In these latter two embodiments, the polymerase may or may not include a3′→5′ exonuclease activity. Also provided are systems and kits forpracticing the subject methods. The subject invention finds use in avariety of different applications, and is particularly suited for use inhigh fidelity PCR based reactions, including SNP detection applications,allelic variation detection applications, and the like.

DEFINITIONS

As used herein, “nucleic acid” means either DNA, RNA, single-stranded ordouble-stranded, and any chemical modifications thereof. Modificationsinclude, but are not limited to, those which provide other chemicalgroups that incorporate additional charge, polarizability, hydrogenbonding, electrostatic interaction, and functionality to the nucleicacid. Such modifications include, but are not limited to, 2′-positionsugar modifications, 5-position pyrimidine modifications, 8-positionpurine modifications, modifications at exocyclic amines, substitution of4-thiouridine, substitution of 5-bromo or 5-iodo-uracil; backbonemodifications, methylations, unusual base-pairing combinations such asthe isobases isocytidine and isoguanidine and the like. Modificationscan also include 3′ and 5′ modifications such as capping.

As used herein, “fluorescent group” refers to a molecule that, whenexcited with light having a selected wavelength, emits light of adifferent wavelength. Fluorescent groups may also be referred to as“fluorophores”.

As used herein, “fluorescence-modifying group” refers to a molecule thatcan alter in any way the fluorescence emission from a fluorescent group.A fluorescence-modifying group generally accomplishes this through anenergy transfer mechanism. Depending on the identity of thefluorescence-modifying group, the fluorescence emission can undergo anumber of alterations, including, but not limited to, attenuation,complete quenching, enhancement, a shift in wavelength, a shift inpolarity, a change in fluorescence lifetime. One example of afluorescence-modifying group is a quenching group.

As used herein, “energy transfer” refers to the process by which thefluorescence emission of a fluorescent group is altered by afluorescence-modifying group. If the fluorescence-modifying group is aquenching group, then the fluorescence emission from the fluorescentgroup is attenuated (quenched). Energy transfer can occur throughfluorescence resonance energy transfer, or through direct energytransfer. The exact energy transfer mechanisms in these two cases aredifferent. It is to be understood that any reference to energy transferin the instant application encompasses all of thesemechanistically-distinct phenomena. Energy transfer is also referred toherein as fluorescent energy transfer or FET.

As used herein, “energy transfer pair” refers to any two molecules thatparticipate in energy transfer. Typically, one of the molecules acts asa fluorescent group, and the other acts as a fluorescence-modifyinggroup. The preferred energy transfer pair of the instant inventioncomprises a fluorescent group and a quenching group. In some cases, thedistinction between the fluorescent group and the fluorescence-modifyinggroup may be blurred. For example, under certain circumstances, twoadjacent fluorescein groups can quench one another's fluorescenceemission via direct energy transfer. For this reason, there is nolimitation on the identity of the individual members of the energytransfer pair in this application. All that is required is that thespectroscopic properties of the energy transfer pair as a whole changein some measurable way if the distance between the individual members isaltered by some critical amount.

“Energy transfer pair” is used to refer to a group of molecules thatform a single complex within which energy transfer occurs. Suchcomplexes may comprise, for example, two fluorescent groups which may bedifferent from one another and one quenching group, two quenching groupsand one fluorescent group, or multiple fluorescent groups and multiplequenching groups. In cases where there are multiple fluorescent groupsand/or multiple quenching groups, the individual groups may be differentfrom one another.

As used herein, “quenching group” refers to any fluorescence-modifyinggroup that can attenuate at least partly the light emitted by afluorescent group. We refer herein to this attenuation as “quenching”.Hence, illumination of the fluorescent group in the presence of thequenching group leads to an emission signal that is less intense thanexpected, or even completely absent. Quenching occurs through energytransfer between the fluorescent group and the quenching group.

As used herein, “fluorescence resonance energy transfer” or “FRET”refers to an energy transfer phenomenon in which the light emitted bythe excited fluorescent group is absorbed at least partially by afluorescence-modifying group. If the fluorescence-modifying group is aquenching group, then that group can either radiate the absorbed lightas light of a different wavelength, or it can dissipate it as heat. FRETdepends on an overlap between the emission spectrum of the fluorescentgroup and the absorption spectrum of the quenching group. FRET alsodepends on the distance between the quenching group and the fluorescentgroup. Above a certain critical distance, the quenching group is unableto absorb the light emitted by the fluorescent group, or can do so onlypoorly.

As used herein “direct energy transfer” refers to an energy transfermechanism in which passage of a photon between the fluorescent group andthe fluorescence-modifying group does not occur. Without being bound bya single mechanism, it is believed that in direct energy transfer, thefluorescent group and the fluorescence-modifying group interfere witheach others electronic structure. If the fluorescence-modifying group isa quenching group, this will result in the quenching group preventingthe fluorescent group from even emitting light.

In general, quenching by direct energy transfer is more efficient thanquenching by FRET. Indeed, some quenching groups that do not quenchparticular fluorescent groups by FRET (because they do not have thenecessary spectral overlap with the fluorescent group) can do soefficiently by direct energy transfer. Furthermore, some fluorescentgroups can act as quenching groups themselves if they are close enoughto other fluorescent groups to cause direct energy transfer. Forexample, under these conditions, two adjacent fluorescein groups canquench one another's fluorescence effectively. For these reasons, thereis no limitation on the nature of the fluorescent groups and quenchinggroups useful for the practice of this invention.

An example of “stringent hybridization conditions” is hybridization at50° C. or higher and 6.0×SSC (900 mM NaCl/90 mM sodium citrate). Anotherexample of stringent hybridization conditions is overnight incubation at42° C. or higher in a solution: 50% formamide, 6×SSC (900 mM NaCl, 90 mMtrisodium citrate), 50 mM sodium phosphate (pH7.6), 10% dextran sulfate,and 20 μg/ml denatured, sheared salmon sperm DNA. Stringenthybridization conditions are hybridization conditions that are at leastas stringent as the above representative conditions, where conditionsare considered to be at least as stringent if they are at least about80% as stringent, typically at least about 90% as stringent as the abovespecific stringent conditions. Other stringent hybridization conditionsare known in the art and may also be employed.

As used herein, the term “nucleic acid intercalator” refers to amolecule that binds to a nucleic acid by inserting itself in betweenbase pairs of adjacent nucleotides without unwinding and withoutextension of the nucleic acid helix. In general, nucleic acidintercalators are aromatic compounds having a flat configuration, andare preferably polycyclic.

As used herein, the term “minor groove binder” refers to a molecule thatbinds to a nucleic acid by inserting itself into the minor groove of aDNA helix. In general, minor groove binders are capable of bindingwithin the minor groove of a DNA helix with an association constant of10³M⁻¹ or greater. In addition, minor groove binders generally havecrescent shaped three-dimensional structures.

As used herein, “3′ end” means at any location on the oligonucleotidefrom and including the 3′ terminus to the center of the oligonucleotide,usually at any location from and including the 3′ terminus to about 10by from the 3′ terminus, and more usually at any location from andincluding the 3′ terminus to about 5 by from the 3′ terminus.

As used herein, “5′ end” means at any location on the oligonucleotidefrom and including the 5′ terminus to the center of the oligonucleotide,usually at any location from and including the 5′ terminus to about 10by from the 5′ terminus, and more usually at any location from andincluding the 5′ terminus to about 5 by from the 5′ terminus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E provide graphical results of assays comparing thefunction of various TET labeled FET probes under high fidelity andstandard PCR conditions.

FIGS. 2A to 2B provide graphical results of assays comparing thefunction of FAM/BHQ1 FET probe and FAM/TAMRA FET probe under highfidelity and standard PCR conditions.

FIG. 3 provides graphical results of a multicomponent analysis of aFAM/TAMRA

FET oligo primer and a FAM/BHQ1 FET oligo primer.

FIG. 4 provides graphical results of assays for allele discriminationusing FAM and TET labeled FET probes under high fidelity PCR conditions.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Methods and compositions are provided for detecting a primer extensionproduct in a reaction mixture. In certain embodiments of subjectmethods, a primer extension reaction is conducted in the presence of apolymerase having exonuclease activity and at least one FET labeledoligonucleotide probe that includes a 3′→5′ exonuclease resistantquencher domain. In certain embodiments, a nucleic acid intercalatorcovalently bonded to said FET labeled oligonucleotide. In certainembodiments, a minor groove binder is employed in the reaction. In theselatter two embodiments, the polymerase employed may or may not haveexonuclease activity. Also provided are systems and kits for practicingthe subject methods. The subject invention finds use in a variety ofdifferent applications, and is particularly suited for use in highfidelity PCR based reactions, including SNP detection applications,allelic variation detection applications, and the like.

Before the subject invention is described further, it is to beunderstood that the invention is not limited to the particularembodiments of the invention described below, as variations of theparticular embodiments may be made and still fall within the scope ofthe appended claims. It is also to be understood that the terminologyemployed is for the purpose of describing particular embodiments, and isnot intended to be limiting. Instead, the scope of the present inventionwill be established by the appended claims.

In this specification and the appended claims, the singular forms “a,”“an” and “the” include plural reference unless the context clearlydictates otherwise. Unless defined otherwise, all technical andscientific terms used herein have the same meaning as commonlyunderstood to one of ordinary skill in the art to which this inventionbelongs.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimit of that range, and any other stated or intervening value in thatstated range, is encompassed within the invention. The upper and lowerlimits of these smaller ranges may independently be included in thesmaller ranges, and are also encompassed within the invention, subjectto any specifically excluded limit in the stated range. Where the statedrange includes one or both of the limits, ranges excluding either orboth of those included limits are also included in the invention.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood to one of ordinary skill inthe art to which this invention belongs. Although any methods, devicesand materials similar or equivalent to those described herein can beused in the practice or testing of the invention, the preferred methods,devices and materials are now described.

All publications mentioned herein are incorporated herein by referencefor the purpose of describing and disclosing those components that aredescribed in the publications that might be used in connection with thepresently described invention.

As summarized above, the subject invention provides methods of detectingthe production of a primer extension product in a primer extensionreaction mixture by using a FET labeled oligonucleotide probe. Infurther describing the subject invention, the methods are describedfirst in greater detail, followed by a review of representative specificapplications in which the methods finds use, as well as systems and kitsthat find use in practicing the subject methods.

Methods

As summarized above, the subject invention provides methods fordetecting the production of primer extension products in a primerextension reaction mixture. In other words, the subject inventionprovides methods of determining whether primer extension products areproduced in a primer extension reaction. By primer extension product ismeant a nucleic acid molecule that results from a template dependentprimer extension reaction. Template dependent primer extension reactionsare those reactions in which a polymerase extends a nucleic acid primermolecule that is hybridized to a template nucleic acid molecule, wherethe sequence of bases that is added to the terminus of the primernucleic acid molecule is determined by the sequence of bases in thetemplate strand. Template dependent primer extension reactions includeboth amplification and non-amplification primer extension reactions. Inmany embodiments of the subject invention, the template dependent primerextension reaction in which the production of primer extension productsis detected is an amplification reaction, e.g., a polymerase chainreaction (PCR).

A feature of certain embodiments of the subject methods is that thetemplate dependent primer extension reaction in which the production ofprimer extension products is detected is a “high fidelity” reaction. By“high fidelity” reaction is meant that the reaction has a low errorrate, i.e., a low rate of wrong nucleotide incorporation. As such, theerror rate of the subject reactions is typically less than about 2×10⁻⁴,usually less than about 1×10⁻⁵ and more usually less than about 1×10⁻⁶.In other embodiments, the reaction mixture is not a high fidelityreaction mixture.

Preparation of High Fidelity Reaction Mixture

In practicing the subject methods of these embodiments, the first stepis to produce a “high fidelity” primer extension mixture, e.g., acomposition of matter that includes all of the elements necessary for ahigh fidelity primer extension reaction to occur, where the primerextension mixture further includes at least one FET labeledoligonucleotide that includes a 3′→5′ exonuclease resistant quencherdomain.

FET occurs when a suitable fluorescent energy donor and an energyacceptor moiety are in close proximity to one another. The excitationenergy absorbed by the donor is transferred to the acceptor which canthen further dissipate this energy either by fluorescent emission if afluorophore, or by non-fluorescent means if a quencher. A donor-acceptorpair comprises two fluorophores having overlapping spectra, where thedonor emission overlaps the acceptor absorption, so that there is energytransfer from the excited fluorophore to the other member of the pair.It is not essential that the excited fluorophore actually fluoresce, itbeing sufficient that the excited fluorophore be able to efficientlyabsorb the excitation energy and efficiently transfer it to the emittingfluorophore.

As such, the FET labeled oligonucleotides employed in the subjectmethods are nucleic acid detectors that include a fluorophore domainwhere the fluorescent energy donor, i.e., donor, is positioned and anacceptor domain where the fluorescent energy acceptor, i.e., acceptor,is positioned. As mentioned above, the donor domain includes the donorfluorophore. The donor fluorophore may be positioned anywhere in thenucleic acid detector, but is typically present at the 5′ terminus ofthe detector.

The acceptor domain includes the fluorescence energy acceptor. Theacceptor may be positioned anywhere in the acceptor domain, but istypically present at the 3′ terminus of the nucleic acid detector.

In addition to the fluorophore and acceptor domains, the FET labeledoligonucleotides also include a target nucleic acid binding domain,which binds to a target nucleic acid sequence, e.g., under stringenthybridization conditions (as defined above). This target binding domaintypically ranges in length from about 10 to about 60 nt, usually fromabout 15 to about 30 nt. Depending on the nature of the oligonucleotideand the assay itself, the target binding domain may hybridize to aregion of the template nucleic acid or a region of the primer extensionproduct. For example, where the assay is a 5′ nuclease assay, e.g., inwhich a Taqman type oligonucleotide probe is employed, the targetbinding domain hybridizes under stringent conditions to a target bindingsite of the template nucleic acid, which is downstream or 3′ of theprimer binding site. In alternative embodiments, e.g., in molecularbeacon type assays, the target binding domain hybridizes to a domain ofa primer extension product.

The overall length of the FET labeled oligonucleotides, which includesall three domains mentioned above, typically ranges from about 10 toabout 60 nt, usually from about 15 to about 30 nt.

The donor fluorophore of the subject probes is typically one that isexcited efficiently by a single light source of narrow bandwidth,particularly a laser source. The emitting or accepting fluorophores areselected to be able to receive the energy from the donor fluorophore andemit light. Usually the donor fluorophores will absorb in the range ofabout 350-800 nm, more usually in the range of about 350-600 nm or500-750 nm. The transfer of the optical excitation from the donor to theacceptor depends on the distance between the two fluorophores. Thus, thedistance must be chosen to provide efficient energy transfer from thedonor to the acceptor. The distance between the donor and acceptormoieties on the FET oligonucleotides employed in the subject invention,at least in certain configurations (such as upon intramolecularassociation) typically ranges from about 10 to about 100 angstroms

The fluorophores for FET pairs may be selected so as to be from asimilar chemical family or a different one, such as cyanine dyes,xanthenes or the like. Fluorophores of interest include, but are notlimited to: fluorescein dyes (e.g., 5-carboxyfluorescein (5-FAM),6-carboxyfluorescein (6-FAM), 2′,4′,1,4,-tetrachlorofluorescein (TET),2′,4′, 5′,7′,1,4-hexachlorofluorescein (HEX), and2′,7′-dimethoxy-4′,5′-dichloro-6-carboxyfluorescein (JOE)), cyanine dyessuch as Cy5, dansyl derivatives, rhodamine dyes (e.g.,tetramethyl-6-carboxyrhodamine (TAMRA), andtetrapropano-6-carboxyrhodamine (ROX)), DABSYL, DABCYL, cyanine, such asCy3, anthraquinone, nitrothiazole, and nitroimidazole compounds, and thelike. Fluorophores of interest are further described in WO 01/42505 andWO 01/86001, as well as the priority U.S. Applications of thesedocuments, the disclosures of the latter of which are hereinincorporated by reference.

A feature of the subject FET labeled oligonucleotides is that they are3′→5′ exonuclease resistant. As such, they are not degraded by 3′→5′exonucleases, i.e., enzymes having exonuclease activity. The 3′→5′exonuclease resistance of the subject FET labeled oligonucleotides mayarise from the presence of the acceptor moiety present in the acceptordomain of the FET labeled oligonucleotide. In many, though not allembodiments, the acceptor moiety is present at the 3′ terminus of theacceptor domain, and in many embodiments at the 3′ terminus of the FETlabeled oligonucleotide as a whole.

Any acceptor or donor that imparts 3′→5′ exonuclease resistance onto theFET labeled oligonucleotides may be employed. In many embodiments, theacceptor moiety is a quencher molecule, e.g., a molecule, that absorbstransferred energy but does not emit fluorescence, e.g., a darkquencher. In many embodiments, the dark quencher has maximum absorbanceof between about 400 and about 700 nm, and often between about 500 andabout 600 nm.

In certain embodiments, the dark quencher comprises a substituted4-(phenyldiazenyl)phenylamine structure, often comprising at least tworesidues selected from aryl, substituted aryl, heteroaryl, substitutedheteroaryl and combination thereof, wherein at least two of saidresidues are covalently linked via an exocyclic diazo bond.

In certain embodiments, the dark quencher is described by the followingformula:

wherein:

-   -   R₀, R₁, R₂, R₃, R₄, R₅ are independently: —H, halogen,        —O(CH₂)_(n)CH₃, —(CH₂)_(n)CH₃, —NO₂, SO₃, —N[(CH₂)_(n)CH₃]₂        wherein n=0 to 5 or —CN;    -   R₆ is —H or —(CH₂)_(n)CH₃ where n=0 to 5; and    -   v is a number from 0 to 10.

Dark quenchers of interest are further described in WO 01/42505 and WO01/86001, as well as the priority U.S. Applications of these documents,the disclosures of the latter of which are herein incorporated byreference.

The FET labeled oligonucleotide may be structured in a variety ofdifferent ways, so long as it includes the above-described donor,acceptor and target nucleic acid binding domains. Typically, the FETlabeled oligonucleotide is structured such that energy transfer occursbetween the fluorophore and acceptor of the FET labeled oligonucleotideprobe upon fluorophore excitation when the FET labeled oligonucleotideis not hybridized to target nucleic acid.

In certain embodiments, the oligonucleotide is a single strandedmolecule that does not form intramolecular structures and in whichenergy transfer occurs because the spacing of the donor and acceptorprovides for energy transfer in the single stranded linear format. Inthese embodiments, energy transfer also occurs between the fluorophoreand acceptor of FET labeled oligonucleotide probe upon fluorophoreexcitation when the FET labeled oligonucleotide probe is hybridized to atarget nucleic acid. Specific examples of such FET labeledoligonucleotide probes include the Taqman™ type probes, as described inU.S. Pat. No. 6,248,526, the disclosure of which is herein incorporatedby reference (as well as Held et al., Genome Res. (1996) 6:986-994;Holland et al., Proc. Nat'l Acad. Sci. USA (1991) 88:7276-7280; and Leeet al., Nuc. Acids Res. (1993) 21:3761-3766 (1993)). In many of theseembodiments, the target nucleic acid binding domain is one thathybridizes to, i.e., is complementary to, a sequence of the templatenucleic acid, i.e., the target nucleic acid of the target nucleic acidbinding domain is a sequence present in the template nucleic acid.

In other embodiments, the probe oligonucleotides are structured suchthat energy transfer does not occur between the fluorophore and acceptorof said FET labeled oligonucleotide probe upon fluorophore excitationwhen the FET labeled oligonucleotide probe is hybridized to a targetnucleic acid. Examples of these types of probe structures include:Scorpion probes (as described in Whitcombe et al., (Nature Biotechnology(1999) 17:804-807; U.S. Pat. No. 6,326,145, the disclosure of which isherein incorporated by reference), Sunrise probes (as described inNazarenko et al., Nuc. Acids Res. (1997) 25:2516-2521; U.S. Pat. No.6,117,635, the disclosure of which is herein incorporated by reference),Molecular Beacons (Tyagi et al., Nature Biotechnology (1996) 14:303-308;U.S. Pat. No. 5,989,823, the disclosure of which is incorporated hereinby reference), and conformationally assisted probes (as described inprovisional application Ser. No. 60/138,376, the disclosure of which isherein incorporated by reference). In many of these embodiments, thetarget binding sequence or domain comprises a hybridization domaincomplementary to a sequence of the primer extension product.

Since the primer extension reaction mixture produced in the initial stepof the subject methods of these particular embodiments is a highfidelity primer extension reaction mixture, it further includes anenzyme having 3′→5′ exonuclease activity. In many embodiments, the 3′→5′exonuclease is a polymerase that has 3′→5′ exonuclease activity. In manyembodiments, the high fidelity nature of the reaction mixture isprovided by the presence of a combination of two or more polymerases, atleast one of which includes a 3′→5′ exonuclease. In certain embodiments,e.g., in 5′ nuclease applications, care is taken to ensure that apolymerase having 5′→3′ nuclease activity is also included. In manyembodiments, the polymerase combination employed includes at least oneFamily A polymerase and, in many embodiments, a Family A polymerase anda Family B polymerase, where the terms “Family A” and “Family B”correspond to the classification scheme reported in Braithwaite & Ito,Nucleic Acids Res. (1993) 21:787-802. Family A polymerases of interestinclude: Thermus aquaticus polymerases, including the naturallyoccurring polymerase (Taq) and derivatives and homologues thereof, suchas Klentaq (as described in Proc. Natl. Acad. Sci. USA (1994)91:2216-2220); Thermus thermophilus polymerases, including the naturallyoccurring polymerase (Tth) and derivatives and homologues thereof, andthe like. Family B polymerases of interest include Thermococcuslitoralis DNA polymerase (Vent) as described in Perler et al., Proc.Natl. Acad. Sci. USA (1992) 89:5577; Pyrococcus species GB-D (DeepVent); Pyrococcus furiosus DNA polymerase (Pfu) as described in Lundberget al., Gene (1991) 108:1-6, Pyrococcus woesei (Pwo) and the like. Ofthe two types of polymerases employed, the Family A polymerase willtypically be present the reaction mixture in an amount greater than theFamily. B polymerase, where the difference in activity will usually beat least 10-fold, and more usually at least about 100-fold. Accordingly,the reaction mixture will typically comprise from about 0.1 U/μl to 1U/μl Family A polymerase, usually from about 0.2 to 0.5 U/μl Family Apolymerase, while the amount of Family B polymerase will typically rangefrom about 0.01 mU/μl to 10 mU/μl, usually from about 0.05 to 1 mU/μland more usually from about 0.1 to 0.5 mU/μl, where “U” corresponds toincorporation of 10 nmoles dNTP into acid-insoluble material in 30 minat 74° C.

In certain embodiments, a nucleic acid intercalator is present in thereaction mixture. In certain of these embodiments, the nucleic acidintercalator is bonded to the FET probe, such that it is stablyassociated with the FET probe under the conditions of use. In many ofthese embodiments, the FET probes employed in the subject inventioninclude a nucleic acid intercalator covalently bonded to the FET labeledoligonucleotide.

The nucleic acid intercalators of interest function to stabilize thenucleic acid helix formed from the FET labeled oligonucleotide. Covalentbinding of nucleic acid intercalators of interest also serve to increasehybrid stability by providing additional binding energy. Furthermore,nucleic acid intercalators of interest provide the FET labeledoligonucleotide greater affinity for its complementary sequence. Inaddition, nucleic acid intercalators provides exonuclease activityresistance when added to the 3′ end of said FET labeled oligonucleotide.The nucleic acid intercalator component of the FET probes of thisembodiment may be located at any location of the FET probe, e.g., at the3′ end or the 5′ end of said FET labeled oligonucleotide, where in manyembodiments the intercalator is covalently bonded to the 3′ end, e.g.,within at least about 10, such as within about 5 nt residues of the 3′end.

In general, nucleic acid intercalators of interest are aromaticcompounds having a flat configuration. They may be cyclic, orpolycyclic, particularly polycyclic aromatic having at least two rings,usually at least three rings and not more than about six rings, moreusually not more than about five rings, where at least two of the ringsare fused, usually at least three of the rings are fused, and usuallynot more than four of the rings being fused. The aromatic compound maybe carbocyclic or heterocyclic, particularly having from one to three,more usually one to two nitrogen atoms as heteroannular atoms. Otherheterannular atoms may include oxygen and sulfur (chalcogen). The ringsmay be substituted by a wide variety of substituents, which substituentsmay include alkyl groups of from one to four carbon atoms, usually fromone to two carbon atoms, oxy, which includes hydroxy, alkoxy and carboxyester, generally of from one to four carbon atoms, amino, includingmono- and disubstituted amino, particularly mono- and dialkyl amino, offrom 0 to 8, usually 0 to 6 carbon atoms, thio, particularly alkylthiofrom 1 to 4, usually 1 to 2 carbon atoms, cyano, non-oxo-carbonyl, suchas carboxy and derivatives thereof, particularly carboxamide orcarboxyalkyl, of from 1 to 8 or 1 to 6 carbon atoms, usually 2 to 6carbon atoms and more usually 2 to 4 carbon atoms, oxo-carbonyl or acyl,generally from 1 to 4 carbon atoms, halo, particularly of atomic number9 to 35, etc.

Polycyclic compounds which find use as nucleic acid intercalatorsinclude acridines, phenanthridines, porphyrins, phenylindoles, andbisbenzamides.

The intercalator may be covalently bonded to the FET labeledoligonucleotide using any convenient protocol, which may or may notemploy a linker group, where representative protocols include thoseprotocols provided herein for covalent bonding of thefluorescer/quencher moieties to the FET labeled probes.

DNA intercalators are further described in U.S. Pat. No. 6,280,933; thedisclosure of which is herein incorporated by reference.

In certain embodiments, another component of the reaction mixtureproduced in the first step of the subject methods is a minor groovebinder. Minor groove binders of interest are those capable of bindingisohelically into the minor groove of double stranded DNA by fittingedge-on into the minor groove, replacing the spine of hydration andfollowing the natural curvature of the DNA. This type of molecularrecognition is at least partially driven by formation of hydrogen bondsbetween the minor groove binder and the DNA. The minor groove binders ofinterest form bifurcated hydrogen bonds with nucleotide base pairs, andnumerous van der Waals contacts with various atoms in the nucleotidebackbone. These atomic interactions stabilize the DNA-minor groovebinder structure and, in turn, effectively strengthen the interaction ofthe two DNA strands of the helix.

In general, minor groove binders of interest are those capable ofbinding within the minor groove of a DNA helix with an associationconstant of 10³M⁻¹ or greater, as determined by using the assaydescribed in Taquet et al., Biochemistry 1998 Jun. 23; 37(25):9119-26.The minor groove binders have a strong preference for A-T (adenine andthymine) rich regions of double stranded DNA. However, minor groovebinders which show preference to C-G (cytosine and guanine) rich regionsare also of interest. Generally, minor groove binders of intereststabilize A-T hydrogen bonds more than C-G bonds, (thus stabilizingweaker sequences). Minor groove binders form extremely stable duplexes

The minor groove binders may be free in the aqueous buffer medium of theprimer extension mixture, or bound to the FET labeled oligonucleotide(covalently or non-covalently). The minor groove binder may be linked toa covalent structure or chain of atoms that attaches the minor groovebinder to the oligonucleotide. This linking chain can and sometimes isconsidered as part of the minor groove binder, and does not adverselyaffect the minor groove binding properties. Furthermore, the minorgroove binders may be attached to either the 3′ end or the 5′ end of theFET labeled oligonucleotide.

In general, minor groove binders have crescent shaped three-dimensionalstructures, and generally have a molecular weight of betweenapproximately 150 to approximately 2000 Daltons. Examples of minorgroove binders include netropsin, distamycin, distamycin A, lexitropsin,mithramycin, chromomycin A₃, olivomycin, anthramycin, sibiromycin,pentamidine, stilbamidine, berenil, CC-1065, Hoechst 33258,4′-6-diamidino-2-phenylindole (DAPI), and derivatives thereof.Typically, the minor groove binder is a N-methylpyrrole peptide such asnetropsin and distamycin A. Minor groove binders are further describedin U.S. Pat. Nos. 5,801,155 and 5,955,590; the disclosures of which areherein incorporated by reference.

In those embodiments where the minor groove binder is present in thereaction mixture, the amount of minor groove binder present in thereaction mixture typically ranges from about 0.02 μM to about 1.5 μM,usually from about 0.1 μM to about 0.75 μM and more usually from about0.2 μM to about 0.5 μM.

Another component of the reaction mixture produced in the first step ofthe subject methods is the template nucleic acid. The nucleic acid thatserves as template may be single stranded or double stranded, where thenucleic acid is typically deoxyribonucleic acid (DNA). The length of thetemplate nucleic acid may be as short as 50 bp, but usually be at leastabout 100 by long, and more usually at least about 150 by long, and maybe as long as 10,000 by or longer, e.g., 50,000 by in length or longer,including a genomic DNA extract, or digest thereof, etc. The nucleicacid may be free in solution, flanked at one or both ends withnon-template nucleic acid, present in a vector, e.g. plasmid and thelike, with the only criteria being that the nucleic acid be availablefor participation in the primer extension reaction. The template nucleicacid may be present in purified form, or in a complex mixture with othernon-template nucleic acids, e.g., in cellular DNA preparation, etc.

The template nucleic acid may be derived from a variety of differentsources, depending on the application for which the PCR is beingperformed, where such sources include organisms that comprise nucleicacids, i.e. viruses; prokaryotes, e.g. bacteria, archaea andcyanobacteria; and eukaryotes, e.g. members of the kingdom protista,such as flagellates, amoebas and their relatives, amoeboid parasites,ciliates and the like; members of the kingdom fungi, such as slimemolds, acellular slime molds, cellular slime molds, water molds, truemolds, conjugating fungi, sac fungi, club fungi, imperfect fungi and thelike; plants, such as algae, mosses, liverworts, hornworts, club mosses,horsetails, ferns, gymnosperms and flowering plants, both monocots anddicots; and animals, including sponges, members of the phylum cnidaria,e.g. jelly fish, corals and the like, combjellies, worms, rotifers,roundworms, annelids, molluscs, arthropods, echinoderms, acorn worms,and vertebrates, including reptiles, fishes, birds, snakes, and mammals,e.g. rodents, primates, including humans, and the like. The templatenucleic acid may be used directly from its naturally occurring sourceand/or preprocessed in a number of different ways, as is known in theart. In some embodiments, the template nucleic acid may be from asynthetic source.

The next component of the reaction mixture produced in the first step ofthe subject methods is the primers employed in the primer extensionreaction, e.g., the PCR primers (such as forward and reverse primersemployed in geometric amplification or a single primer employed in alinear amplification). The oligonucleotide primers with which thetemplate nucleic acid (hereinafter referred to as template DNA forconvenience) is contacted will be of sufficient length to provide forhybridization to complementary template DNA under annealing conditions(described in greater detail below) but will be of insufficient lengthto form stable hybrids with template DNA under polymerizationconditions. The primers will generally be at least 10 by in length,usually at least 15 by in length and more usually at least 16 by inlength and may be as long as 30 by in length or longer, where the lengthof the primers will generally range from 18 to 50 by in length, usuallyfrom about 20 to 35 by in length. The template DNA may be contacted witha single primer or a set of two primers (forward and reverse primers),depending on whether primer extension, linear or exponentialamplification of the template DNA is desired. Where a single primer isemployed, the primer will typically be complementary to one of the 3′ends of the template DNA and when two primers are employed, the primerswill typically be complementary to the two 3′ ends of the doublestranded template DNA.

In addition to the above components, the reaction mixture produced inthe subject methods includes deoxyribonucleoside triphosphates (dNTPs).Usually the reaction mixture will comprise four different types of dNTPscorresponding to the four naturally occurring bases are present, i.e.dATP, dTTP, dCTP and dGTP. In the subject methods, each dNTP willtypically be present in an amount ranging from about 10 to 5000 μM,usually from about 20 to 1000 μM.

The reaction mixture prepared in the first step of the subject methodsfurther includes an aqueous buffer medium that includes a source ofmonovalent ions, a source of divalent cations and a buffering agent. Anyconvenient source of monovalent ions, such as KCl, K-acetate,NH₄-acetate, K-glutamate, NH₄Cl, ammonium sulfate, and the like may beemployed. The divalent cation may be magnesium, manganese, zinc and thelike, where the cation will typically be magnesium. Any convenientsource of magnesium cation may be employed, including MgCl₂, Mg-acetate,and the like. The amount of Mg²⁺ present in the buffer may range from0.5 to 10 mM, but will preferably range from about 3 to 6 mM, and willideally be at about 5 mM. Representative buffering agents or salts thatmay be present in the buffer include. Tris, Tricine, HEPES, MOPS and thelike, where the amount of buffering agent will typically range fromabout 5 to 150 mM, usually from about 10 to 100 mM, and more usuallyfrom about 20 to 50 mM, where in certain preferred embodiments thebuffering agent will be present in an amount sufficient to provide a pHranging from about 6.0 to 9.5, where most preferred is pH 7.3 at 72° C.Other agents which may be present in the buffer medium include chelatingagents, such as EDTA, EGTA and the like.

In preparing the reaction mixture, the various constituent componentsmay be combined in any convenient order. For example, the buffer may becombined with primer, polymerase and then template DNA, or all of thevarious constituent components may be combined at the same time toproduce the reaction mixture.

Preparation of Non-High Fidelity Reaction Mixture

As indicated above, in certain embodiments, e.g., where a DNAintercalator and/or a minor groove binder is present, the reactionmixture need not be a high fidelity reaction mixture. In theseembodiments, the reaction mixture is prepared as described above, butthe polymerase having exonuclease activity, e.g., the Family Bpolymerases reviewed above, is not included in the reaction mixture.

Subjecting the Primer Extension Mixture to Primer Extension ReactionConditions

Following preparation of the reaction mixture, the reaction mixture issubjected to primer extension reaction conditions, i.e., to conditionsthat permit for polymerase mediated primer extension by addition ofnucleotides to the end of the primer molecule using the template strandas a template. In many embodiments, the primer extension reactionconditions are amplification conditions, which conditions include aplurality of reaction cycles, where each reaction cycle comprises: (1) adenaturation step, (2) an annealing step, and (3) a polymerization step.The number of reaction cycles will vary depending on the applicationbeing performed, but will usually be at least 15, more usually at least20 and may be as high as 60 or higher, where the number of differentcycles will typically range from about 20 to 40. For methods where morethan about 25, usually more than about 30 cycles are performed, it maybe convenient or desirable to introduce additional polymerase into thereaction mixture such that conditions suitable for enzymatic primerextension are maintained.

The denaturation step comprises heating the reaction mixture to anelevated temperature and maintaining the mixture at the elevatedtemperature for a period of time sufficient for any double stranded orhybridized nucleic acid present in the reaction mixture to dissociate.For denaturation, the temperature of the reaction mixture will usuallybe raised to, and maintained at, a temperature ranging from about 85 to100, usually from about 90 to 98 and more usually from about 93 to 96°C. for a period of time ranging from about 3 to 120 sec, usually fromabout 5 to 30 sec.

Following denaturation, the reaction mixture will be subjected toconditions sufficient for primer annealing to template DNA present inthe mixture. The temperature to which the reaction mixture is lowered toachieve these conditions will usually be chosen to provide optimalefficiency and specificity, and will generally range from about 50 to75, usually from about 55 to 70 and more usually from about 60 to 68° C.Annealing conditions will be maintained for a period of time rangingfrom about 15 sec to 30 min, usually from about 30 sec to 5 min.

Following annealing of primer to template DNA or during annealing ofprimer to template DNA, the reaction mixture will be subjected toconditions sufficient to provide for polymerization of nucleotides tothe primer ends in manner such that the primer is extended in a 5′ to 3′direction using the DNA to which it is hybridized as a template, i.e.,conditions sufficient for enzymatic production of primer extensionproduct. To achieve polymerization conditions, the temperature of thereaction mixture will typically be raised to or maintained at atemperature ranging from about 65 to 75, usually from about 67 to 73° C.and maintained for a period of time ranging from about 15 sec to 20 min,usually from about 30 sec to 5 min.

The above cycles of denaturation, annealing and polymerization may beperformed using an automated device, typically known as a thermalcycler. Thermal cyclers that may be employed are described in U.S. Pat.Nos. 5,612,473; 5,602,756; 5,538,871; and 5,475,610, the disclosures ofwhich are herein incorporated by reference.

Signal Detection

The next step in the subject methods is signal detection, i.e.,detecting a change in a fluorescent signal from the FET labeledoligonucleotide probe present in the reaction mixture to obtain an assayresult. In other words, the next step in the subject methods is todetect any modulation in the fluorescent signal generated by the FETlabeled oligonucleotide present in the reaction mixture. The change maybe an increase or decrease in fluorescence, depending on the nature ofthe label employed, but in many embodiments is an increase influorescence. The sample may be screened for an increase in fluorescenceusing any convenient means, e.g., a suitable fluorimeter, such as athermostable-cuvette or plate-reader fluorimeter. Fluorescence issuitably monitored using a known fluorimeter. The signals from thesedevices, for instance in the form of photo-multiplier voltages, are sentto a data processor board and converted into a spectrum associated witheach sample tube. Multiple tubes, for example 96 tubes, can be assessedat the same time. Data may be collected in this way at frequentintervals, for example once every 10 ms, throughout the reaction. Bymonitoring the fluorescence of the reactive molecule from the sampleduring each cycle, the progress of the amplification reaction can bemonitored in various ways. For example, the data provided by meltingpeaks can be analyzed, for example by calculating the area under themelting peaks and this data plotted against the number of cycles.

The spectra generated in this way can be resolved, for example, using“fits” of pre-Selected fluorescent moieties such as dyes, to form peaksrepresentative of each signaling moiety (i.e. fluorophore). The areasunder the peaks can be determined which represents the intensity valuefor each signal, and if required, expressed as quotients of each other.The differential of signal intensities and/or ratios will allow changesin FET to be recorded through the reaction or at different reactionconditions, such as temperatures. The changes are related to the bindingphenomenon between the oligonucleotide probe and the target sequence ordegradation of the oligonucleotide probe bound to the target sequence.The integral of the area under the differential peaks will allowintensity values for the FET effects to be calculated.

Screening the mixture for a change in fluorescence provides one or moreassay results, depending on whether the sample is screened once at theend of the primer extension reaction, or multiple times, e.g., aftereach cycle, of an amplification reaction (e.g., as is done in real timePCR monitoring).

Employing Said Assay Result to Determine Whether a Primer ExtensionProduct is Present in Said Mixture

The data generated as described above can be interpreted in variousways. In its simplest form, an increase or decrease in fluorescence fromthe sample in the course of or at the end of the amplification reactionis indicative of an increase in the amount of the target sequencepresent, i.e., primer extension product present, suggestive of the factthat the amplification reaction has proceeded and therefore the targetsequence was in fact present in the sample. Quantitation is alsopossible by monitoring the amplification reaction throughout theamplification process.

In this manner, a reaction mixture is readily screened for the presenceof primer extension products. The methods are suitable for detection ofa single primer extension product as well as multiplex analyses, inwhich two or more different FET labeled oligonucleotide probes areemployed to screen for two or more different primer extension products.In these latter multiplex situations, the number of different types ofprobes that may be employed typically ranges from about 2 to about 20 orhigher, usually from about 2 to about 15.

The above described methods of detecting the presence of one or moretypes of primer extension reaction products in a primer extensionreaction mixture finds use in a variety of different applications,representative ones of which are now reviewed in greater detail.

Utility

The above-described inventive methods find use in a variety of differentapplications. In general, the subject oligonucleotide probes and methodsof using the same find use in any high fidelity primer extensionreaction in which a FET probe and proofreading polymerase are employed.

One type of representative application is in monitoring the progress ofnucleic acid amplification reactions, such as polymerase chain reactionapplications, including both linear and geometric PCR applications. Asused herein, the term monitoring includes a single evaluation at the endof a series of reaction cycles as well as multiple evaluations, e.g.,after each reaction cycle, such that the methods can be employed todetermine whether a particular amplification reaction series hasresulted in the production of primer extension product, e.g., a non-realtime evaluation, as well as in a real-time evaluation of the progress ofthe amplification reaction.

The subject methods find use in both 5′ nuclease methods of monitoring aPCR amplification reaction (e.g., where a Taqman type probe isemployed); and non-5′ nuclease methods of monitoring a PCR amplificationreaction (e.g., where a molecular beacon type probe is employed). Again,the subject methods find use in evaluating the progress of anamplification reaction at a single time (e.g., non-real time monitoring)and in real-time monitoring.

Monitoring a PCR reaction according to the subject methods finds use ina variety of specific applications. Representative applications ofinterest include, but are not limited to: (1) detection of allelicpolymorphism; (2) SNP detection; (3) detection of rare mutations; (4)detection of allelic stage of single cells; (5) detection of single orlow copy number DNA analyte molecules in a sample; etc. For example, indetection of allelic polymorphism, a nucleic acid sample to be screened,e.g., a genomic DNA cellular extract, is employed as template nucleicacid in the preparation of a primer extension reaction mixture, asdescribed above, where the reaction mixture includes a different anddistinguishable FET labeled oligonucleotide probe that is specific foreach different allelic sequence to be identified, if present. The assayis then carried out as described above, where the sample is screened fora change in signal from each different oligonucleotide probe. A changein signal from a given probe is indicative of the presence the allelicvariant to which that probe is specific in the sample. Likewise, anabsence of change in signal is indicative of the absence of the allelicvariant in the sample. In this manner, the sample is readily screenedfor the presence of one or more allelic variants. A similar approach canbe used for SNP detection, where a different FET labeled oligonucleotidefor each SNP of interest to be screened in a nucleic acid sample isemployed.

A significant benefit of employing the subject methods in PCR screeningapplications is that the PCR conditions may be “high fidelity,” i.e.,they may include a proof reading activity, such that the resultsobtained from the assays performed according to the subject methods arehighly reliable.

Kits

Also provided are kits for practicing the subject methods. The kitsaccording to the present invention will comprise at least: (a) a FETlabeled oligonucleotide, where the kits may include two or moredistinguishable FET labeled oligonucleotides, e.g., that hybridize todifferent target nucleic acids, e.g., two or more different SNPs; and(b) instructions for using the provide FET labeled oligonucleotide(s) ina high fidelity amplification, e.g., PCR, reaction.

The subject kits may further comprise additional reagents which arerequired for or convenient and/or desirable to include in the reactionmixture prepared during the subject methods, where such reagentsinclude: one or more polymerases, including a polymerase mix, where theone or more polymerases at least include a polymerase that exhibitsproofreading, i.e., 3′→5′ exonuclease activity in certain embodiments;an aqueous buffer medium (either prepared or present in its constituentcomponents, where one or more of the components may be premixed or allof the components may be separate), and the like.

The various reagent components of the kits may be present in separatecontainers, or may all be precombined into a reagent mixture forcombination with template DNA. For example, the kit may include anintercalator-FET probe and a minor groove binder, where these twocomponents may be present separately or combined into a singlecomposition for use.

In addition to the above components, the subject kits will furtherinclude instructions for practicing the subject methods. Theseinstructions may be present in the subject kits in a variety of forms,one or more of which may be present in the kit. One form in which theseinstructions may be present is as printed information on a suitablemedium or substrate, e.g., a piece or pieces of paper on which theinformation is printed, in the packaging of the kit, in a packageinsert, etc. Yet another means would be a computer readable medium,e.g., diskette, CD, etc., on which the information has been recorded.Yet another means that may be present is a website address which may beused via the internet to access the information at a removed site. Anyconvenient means may be present in the kits.

Systems

Also provided are systems for use in practicing the subject methods. Thesubject systems at least include one or more FET labeledoligonucleotides and a proofreading activity, as well as any otherrequisite components for preparing a primer extension reaction mixture,as described above. In addition, the subject systems may include anyrequired devices for practicing the subject methods, e.g., thermalcyclers, fluorimeters, etc.

The following examples are offered by way of illustration and not by wayof limitation.

EXPERIMENTAL PROCEDURES A. General

The oligonucleotides shown in Table 1 were used for the followingexample. Standard DNA phosphoramidites, including 6-carboxy-fluorescein(6-FAM) phosphoramidite, 5′-Tetrachloro-Fluorescein (TET)phosphoramidite, and 6-carboxytetramethyl-rhodamine (TAMRA) CPG,3′-Dabcyl CPG, were obtained from Glen Research. Black Hole Quenchers(BHQ1, BHQ2, BHQ3) CPG were obtained from Biosearch Technology. EclipseDark Quencher (eDQ) CPG was obtained from Epoch Bioscience. All primerswere purified using Oligo Purification Cartridges (BiosearchTechnology). Doubly labeled FET probes were synthesized using CPGs withvarious quenchers as indicated in Table 1 and with either 6′FAM-labeledor TET-labeled phosphoramidites at the 5′ end. The doubly labeled FETprobes were purified by preparative HPLC and PAGE using standardprotocols. Phosphorothioate modification was prepared by standardprocedure.

TABLE I Sequence Listing Name Type Sequence BCL-F1 SEQ ID NO: 1 Primer5′ GGT GGT GGA GGA GCT CTT CAG 3′ BCL-R1 SEQ ID NO: 2 Primer 5′CCA GCC TCC GTT ATC CTG GA 3′ BCL-P1 SEQ ID NO: 3 Probe 5′FAM-CCT GTG GAT GAC TGA GTA CCT GAA CCG- BHQ1-3′ BCL-P2 SEQ ID NO: 4Probe 5′ FAM-CCT GTG GAT GAC TGA GTA CCT GAA CCG- eDQ-3′ BCL-P3SEQ ID NO: 5 Probe 5′ FAM-CCT GTG GAT GAC TGA GTA CCT GAA CCG- DABCYL-3′BCL-P4 SEQ ID NO: 6 Probe 5′ FAM-CCT GTG GAT GAC TGA GTA CCT GAA CCG-TAMRA-3′ BCL-P5 SEQ ID NO: 7 S-oligo 5′ FAM-CCT GTG GAT GAC TGA GTA CCTProbe GAA*C*C*G-TAMRA-3′ ACT-F1 SEQ ID NO: 8 Primer 5′GAG CTA CGA GCT GCC TGA C 3′ ACT-R1 SEQ ID NO: 9 Primer 5′GAC TCC ATG CCC AGG AAG 3′ ACT-P1 SEQ ID NO: 10 Probe 5′TET-CAT CAC CAT TGG CAA TGA GCG-BHQ1-3′ ACT-P2 SEQ ID NO: 11 Probe 5′TET-CAT CAC CAT TGG CAA TGA GCG-eDQ-3′ ACT-P3 SEQ ID NO: 12 Probe 5′TET-CAT CAC CAT TGG CAA TGA GCG-DABCYL-3′ ACT-P4 SEQ ID NO: 13 Probe 5′TET-CAT CAC CAT TGG CAA TGA GCG-TAMRA-3′ ACT-P5 SEQ ID NO: 14 S-oligo 5′TET-CAT CAC CAT TGG CAA TGA *G*C*G-TAMRA- Probe 3′ ABCG-R1 SEQ ID NO: 15Primer 5′ CCC AAA AAT TCA TTA TGC TGC AA 3' ABCG-P1 SEQ ID NO: 16 Primer5′ FAM-CAG CAT TCC ACG ATA TGG ATT TAC GGC- BHQ1-3′ ABCG-P2SEQ ID NO: 17 Primer 5′ FAM-CAG CAT TCC ACG ATA TGG ATT TAC GGC-TAMRA-3′ ABCG-T1 SEQ ID NO: 18 Template 5′ATC AGC ATT CCA CGA TAT GGA TTT ACG GCATCA GTT GCA GCA TAA TGA ATT TTT GGG A 3′ MTHFR-F1 SEQ ID NO: 19 Primer5′ GGA AGA ATG TGT CAG CCT CAA AG 3′ MTHFR-R1 SEQ ID NO: 20 Primer 5′CTG ACC TGA AGC ACT TGA AGG AG 3′ MTHFR-P1 SEQ ID NO: 21 Wt Probe 5′TET- TGA AAT CGG CTC CCG CA -BHQ1-3′ MTHFR-P2 SEQ ID NO: 22 Mut Probe 5′FAM- TGA AAT CGA CTC CCG CAG A -BHQ1-3′ *phosphorothioateinternucleotide linkage

Example 1 Properties and Use of TET Labeled FET Probes with VariousQuenchers

Real Time amplifications and detection were performed in ABI PRISM 7700

(Applied Biosystems) using 30 μl reactions that contained 20 mM Tris-HCl(pH8.3), 60 mM KCl, 5.3 mM MgCl₂, 0.2 mM dATP, 0.2 mM dTTP, 0.2 mM dGTP,0.2 mM dCTP, 0.5 μM of each primer, 0.2 μM FET probe, 4-fold seriesdilute of Jurkat cDNAs, either 0.6 units Taq Polymerase (Fisher) or amix of Taq and Pwo DNA Polymerase (Roche Molecular Biochemicals) at unitratio of 5 to 1.

A 99 basepair segment of the human beta actin gene was amplified usingprimers ACT-F1 and ACT-R1 listed in Table 1. Thermal profile was 95° C.15 sec; 50 cycles of 95° C. 15 sec, 60° C. 30 sec, 72° C. 45 sec. Afteramplification, fluorescent intensity at each well was measured by postPCR reading.

Five types of TET labeled FET probes (ACT-P1, ACT-P2, ACT-P3, ACT-P4 andACT-P5 as shown in Table 1) were tested in Real Time amplification. PostPCR reading result is shown in Table 2. The emission intensity of donorunder condition with template is divided by the emission intensity ofdonor under condition without template to give +/− signal ratio, whichindicates whether the probes are degraded or not. For TET labeled FETprobes having TAMRA or Dabcyl as quencher, 3′→5′ exonuclease additioncaused a dramatic decrease in +/− signal ratio due to cleavage of FETprobes.

TABLE 2 Fluorescent changes in various TET labeled FET probe during PCRPCR using Taq pol. PCR using Taq + Pwo pol. donor/quencher no templateTemplate +/−signal ratio no template template +/−signal ratio TET/BHQ14500.0 20250.0 4.5 4500.0 20000.0 4.4 TET/eDQ 4000.0 20000.0 5.0 4000.020000.0 5.0 TET/Dabcyl 6000.0 19000.0 3.2 16000.0 24000.0 1.5 TET/TAMRA3900.0 19000.0 4.9 12000.0 18000.0 1.5 TET/TAMRA S-oligo 4000.0 20000.05.0 3900.0 19000.0 4.9

Real Time amplification plot is shown in FIG. 1A-E. TET labeled FETprobes having either BHQ1 or Eclipse Dark Quencher at 3′-end areresistant to 3′→5′ exonuclease, and are therefore suitable for use inhigh fidelity PCR (FIGS. 1A and 1B). By contrast, TET labeled FET probeshaving either TAMRA or Dabcyl Quencher at 3′-end are degraded by 3′→5′exonuclease, and no proper real time PCR results can be obtained (FIG.1C and 1D). The TET labeled FET probes having TAMRA Quencher at 3′-endcan also be resistant to 3′→5′ exonuclease if three phosphorothioateinternucleotide linkages at 3′-end are added (FIG. 1E), which confirmsthe reliability of this whole experiment.

Example 2 Properties and Use of FAM Labeled FET Probes with VariousQuenchers

Real Time amplifications and detection were performed in ABI PRISM 7700using 30 μl reactions that contained 20 mM Tris-HCl (pH8.3), 60 mM KCl,5.3 mM MgCl₂, 0.2 mM dATP, 0.2 mM dTTP, 0.2 mM dGTP, 0.2 mM dCTP, 0.5 uMof each primers, 0.2 μM FET probe, various copies of a cDNA clone ofhuman bcl-2 gene or Jurkat cDNAs, 0.6 units Taq DNA Polymerase (FisherScientific) or a mix of Taq and Pwo DNA Polymerase (Roche MolecularBiochemicals) at unit ratio of 10 to 1.

A 190 basepair segment of the human bcl-2 gene was amplified usingprimers BCL-F1 and BCL-R1 listed in Table 1. Thermal profile was 95° C.15 sec; 50 cycles of 95° C. 15 sec, 60° C. 30 sec, 72° C. 45 sec. Afteramplification, fluorescent intensity at each well was measured by postPCR reading.

Five types of FAM-labeled FET probes (BCL-P1, BCL-P2, BCL-P3, BCL-P4 andBCL-P5 as shown in Table 1) were tested in Real Time amplification. PostPCR reading result is shown in Table 3. For FAM labeled FET probeshaving TAMRA or Dabcyl as quencher, 3′→5′ exonuclease addition caused adramatic decrease in +/− signal ratio due to cleavage of FET probes.

TABLE 3 Fluorescent changes in various FAM labeled FET probe during PCRPCR using Taq pol. PCR using Taq + Pwo pol. donor/quencher no templatetemplate +/−signal ratio no template template +/−signal ratio FAM/BHQ15000.0 20000.0 4.0 5000.0 20000.0 4.0 FAM/eDQ 5000.0 20000.0 4.0 5000.019000.0 3.8 FAM/Dabcyl 7000.0 20000.0 2.9 18000.0 25000.0 1.4 FAM/TAMRA5000.0 22500.0 4.5 20000.0 30000.0 1.5 FAM/TAMRA S-oligo 10000.0 30000.03.0 10000.0 29000.0 2.9

Representative Real Time amplification plot is shown in FIG. 2A-2B. FAMlabeled FET probes having either BHQ1 or Eclipse Dark Quencher at 3′-endare resistant to 3′→5′ exonuclease, and are therefore suitable for usein high fidelity PCR (FIG. 2A). By contrast, FAM labeled FET probeshaving either TAMRA or Dabcyl Quencher at 3′-end are degraded by 3′→5′exonuclease, and no proper real time PCR results can be obtained (FIG.2B). Thus, similar results as Example 1 are repeated in a differentsystem.

Example 3 Properties of FET Oligo as Primers

PCR amplifications were performed in ABI PRISM 7700 using 30 μlreactions that contained 10 mM Tris-HCl (pH8.85), 25 mM KCl, 5 mM(NH₄)₂SO₄, 5 mM MgCl₂, 0.2 mM dATP, 0.2 mM dTTP, 0.2 mM dGTP, 0.2 mMdCTP, 0.5 μM of each primer, 0.2 μM of each primer (ABCG-R1 and ABCG-P1or ABCG-P2), 1 million copies of template (ABCG-T1) or just TE buffer inno template control (NTC), 0.6 units of Pwo DNA Polymerase (RocheMolecular Biochemicals). Thermal profile was 95° C. 10 sec; 40 cycles of95° C. 15 sec, 60° C. 60 sec.

Two types of FET primers (ABCG-P1 and ABCG-P2) and a template (ABCG-T1)as listed in Table 1 were tested in Real Time Amplification of human ABCtransporter ABCG2. Multicomponent analysis is shown in FIG. 3. Thus, thesubject FET labeled nucleic acid detectors having a BHQ1 at 3′-end areresistant to exonuclease activity when used as a primer. By contrast,the FET labeled nucleic acid detectors having a TAMRA at 3′-end aredegraded by exonuclease activity. Similar results were obtained usingFET primers having mismatch at 3′-end.

Example 4 Allele Discrimination Under High Fidelity PCR

An 89 basepair segment of the human methylenetetrahydrofolate reductase(MTHFR) gene was amplified using primers MTHFR-F1 and MTHFR-R1 listed inTable 1. Real Time amplifications and detection were performed in ABIPRISM 7700 using 30 μl reactions that contained 20 mM Tris-HCl (pH8.3),60 mM KCl, 5.3 mM MgCl₂, 0.2 mM dATP, 0.2 mM dTTP, 0.2 mM dGTP, 0.2 mMdCTP, 0.1 uM of primer MTHFR-F1 and 1 μM of primer MTHFR-R1, 0.2 μM eachof FET probes MTHFR-P1 and MTHFR-P2, templates containing varies numberof MTHFR wild type or mutant or both as listed in table 4, a mix of Taqand Pwo DNA Polymerase (Roche Molecular Biochemicals) at unit ratio of 5to 1. Thermal profile was 95° C. 15 sec; 50 cycles of 95° C. 15 sec, 55°C. 40 sec, 70° C. 40 sec. After amplification, fluorescent intensity ateach well was measured by post PCR reading and analyzed by allelediscrimination software in ABI PRISM 7700. Graphical result is shown inFIG. 4 and further summarized in Table 4.

TABLE 4 Summary of template types and allele call by PRISM 7700 Templatetypes Call by Template types Call by Template types Call by Templatetypes Call by (copy num) PRISM 7700 (copy num) PRISM 7700 (copy num)PRISM 7700 (copy num) PRISM 7700 mut: 10⁶ Allele 2 mut: 2 × 10⁵ Allele1/2 mut: 8 × 10³ Allele 1 Wt: 10⁶ Allele 1 wt: 10⁵ wt: 10⁵ mut: 10⁶Allele 2 mut: 2 × 10⁵ Allele 1/2 mut: 8 × 10³ Allele 1 Wt: 10⁶ Allele 1wt: 10⁵ wt: 10⁵ mut: 10⁶ Allele 2 mut: 2 × 10⁵ Allele 1/2 mut: 8 × 10³Allele 1 Wt: 10⁶ Allele 1 wt: 10⁵ wt: 10⁵ mut: 10⁶ Allele 2 mut: 2 × 10⁵Allele 1/2 mut: 8 × 10³ Allele 1 Wt: 10⁶ Allele 1 wt: 10⁵ wt: 10⁵ mut:10⁶ Allele 2 mut: 4 × 10⁴ Allele 1/2 mut: 1.6 × 10³ Allele 1 No templateNo amp wt: 10⁵ wt: 10⁵ wt: 10⁵ Mut: 10⁶ Allele 2 mut: 4 × 10⁴ Allele 1/2mut: 1.6 × 10³ Allele 1 No template No amp wt: 10⁵ wt: 10⁵ wt: 10⁵ mut:10⁶ Allele 2 mut: 4 × 10⁴ Allele 1/2 mut: 1.6 × 10³ Allele 1 No templateNo amp wt: 10⁵ wt: 10⁵ wt: 10⁵ mut: 10⁶ Allele 2 mut: 4 × 10⁴ Allele 1/2mut: 1.6 × 10³ Allele 1 No template No amp wt: 10⁵ wt: 10⁵ wt: 10⁵Allele 1 = wild type (TET labeled probe) Allele 2 = mutant type (FAMlabeled probe)

The call by ABI PRISM 7700 matches well with types of template used(Table 4). No template controls give no amplification signal. Thus, twodifferent allele sequences can be detected and distinguished using FAMand TET labeled FET probes under high fidelity PCR.

B. General

The oligonucleotides shown in Table 5 were used for the followingexample. Standard DNA phosphoramidites, including 6-carboxy-fluorescein(6-FAM) phosphoramidite, 5′-Tetrachloro-Fluorescein (TET)phosphoramidite, Acridine (ACR) phosphoramidite and6-carboxytetramethyl-rhodamine (TAMRA) CPG, 3′-Dabcyl CPG, were obtainedfrom Glen Research. Black Hole Quenchers (BHQ1) CPG were obtained fromBiosearch Technology. Eclipse Dark Quencher (eDQ) CPG was obtained fromEpoch Bioscience. All primers were purified using Oligo PurificationCartridges (Biosearch Technology). Doubly labeled FET probes weresynthesized using CPGs with various quenchers as indicated in Table 5and with either 6′FAM-labeled or TET-labeled phosphoramidites at the 5′end. The doubly labeled FET probes were purified by preparative HPLC andPAGE using standard protocols. Phosphorothioate modification wasprepared by standard procedure. Minor groove binder netropsin anddistamycin A were purchased from Sigma.

TABLE 5 (continue) Name Type Sequence MTHFR-P1 SEQ ID NO: 21 Wt Probe 5′TET- TGA AAT CGG CTC CCG CA -BHQ1-3′ MTHFR-P2 SEQ ID NO: 22 Mut Probe 5′FAM- TGA AAT CGA CTC CCG CAG A -BHQ1-3′ MTHFR-P13 SEQ ID NO: 23 Wt Probe5′ TET- TGA AAT CGG CTC CCG C -ACR-BHQ1-3′ MTHFR-P14 SEQ ID NO: 24Wt Probe 5′ TET-ACR TGA AAT CGG CTC CCG C -BHQ1-3′ MTHFR-P3SEQ ID NO: 25 Mut Probe 5′ FAM- TGA AAT CGA CTC CCG CAG -ACR-BHQ1-3′

Example 6 Allele Discrimination Using Acridine-Labeled FET Probes UnderHigh Fidelity PCR

An 89 basepair segment of the human methylenetetrahydrofolate reductase(MTHFR) gene was amplified using primers MTHFR-F1 and MTHFR-R1 listed inTable 1. Real Time amplifications and detection were performed in ABIPRISM 7700 using 30 μl reactions that contained 20 mM Tris-HCl (pH8.3),60 mM KCl, 5.3 mM MgCl₂, 0.2 mM dATP, 0.2 mM dTTP, 0.2 mM dGTP, 0.2 mMdCTP, 0.15 uM of reference dye ROX, 0.5 uM each of primers MTHFR-F1 andMTHFR-R1, 0.2 μM each of FET probes, templates containing varies numberof MTHFR wild type or mutant or both as listed in Table 6, a mix of Taqand Pwo DNA Polymerase (Roche Molecular Biochemicals) at unit ratio of 5to 1. Five types of FET probes (MTHFR-P1, MTHFR-P2, MTHFR-P3, MTHFR-P13and MTHFR-P14 as shown in Table 5) were tested in Real Timeamplification. Thermal profile was 95° C. 15 sec; 50 cycles of 95° C. 15sec, 57° C. 40 sec, 70° C. 40 sec.

Fluorescent intensity at each well was analyzed using real time data andsoftware in ABI PRISM 7700 and summarized in Table 6. There wasconsiderable cross-reactivity (or noise) between dyes FAM and TET (32%and 46% respectively) when normal FET probes were used. Suchcross-reactivity was reduced to 2% and 4% when 3′-ACR labeled FET probeswere used. However, 5′-ACR labeled FET probes did not show muchdifference to normal FET probes.

TABLE 6 Summary of probe and template types and ΔRn changesTemplate types (copy num) wt: 10⁶ Mut: 10⁶ Fluorescent Intensity (ΔRn)Probe Types TET ΔRn FAM ΔRn % noise 5′FAM- TGA AAT CGA CTC CCG CAG -ACR-BHQ1-3′ SEQ. ID No. 25 0.1 4.1  2% 5′FAM- TGA AAT CGA CTC CCG CAG A -BHQ1-3′ SEQ. ID No. 22 1.4 4.4 32% 5′TET- TGA AAT CGG CTC CCG C -ACR-BHQ1-3′ SEQ. ID No. 23 4.5 0.2  4% 5′TET-ACR- TGA AAT CGG CTC CCG C -BHQ1-3′ SEQ. ID No. 24 2.7 0.8 30% 5′TET- TGA AAT CGG CTC CCG CA -BHQ1-3′ SEQ. ID No. 21 5.4 2.5 46%

Real Time amplifications and detection were further performed under thesame conditions as above in ABI PRISM 7700 using 0.2 μM each of eithernormal or 3′-ACR-labeled FET probes as shown in Table 7 for allelediscrimination. Real Time thermal profile was 95° C. 15 sec; 50 cyclesof 95° C. 15 sec, 57° C. 40 sec, 70° C. 40 sec.

Fluorescent intensity of each dye at each well was analyzed using realtime data and software in ABI PRISM 7700 and summarized in Table 6.There was considerable cross-reactivity (or noise) between dyes FAM andTET when normal FET probes were used. Such cross-reactivity was reducedto ˜2% when 3′-ACR labeled FET probes were used. Because of much lesscross-reactivity (or noise) between wt (TET) and mut (FAM) probes whenusing 3′-ACR-labeled FET probes, allelic analysis using 3′ACR-labeledprobes was much closer to the theoretical prediction, therefore moreaccurate than using normal FET probe.

TABLE 7Summary of probe and template types and allele discrimination resultsTemplate types (copy num) wt: mut: wt: 10⁶ + wt: 10⁵ + wt: 10⁵ + 10⁶ 10⁶mut: 10⁶ mut: 10⁵ mut: 10⁶ Probe Types Fluorescent Intensty (ΔRn) 5′FAM- TGA AAT CGA CTC CCG CAG  FAM ΔRn 0.7 5.4 3.8 1.4 5.3 A -BHQ1-3′(SEQ ID NO: 22) + 5′ TET- TGA AAT CGG CTC CCG CA -BHQ1-3′ TET ΔRn 5.30.4 3.2 5.2 1.3 (SEQ ID NO: 21) % of dominant allele /Rn to total ΔRn 88%  93% 54% 79% 80% Theoretical % 100% 100% 50% 91% 91%5′FAM- TGA AAT CGA CTC CCG  FAM ΔRn 0.1 4.9 3.1 0.9 5.3 CAG -ACR-BHQ1-3′(SEQ ID NO: 25) + 5′ TET- TGA AAT CGG CTC CCG C -ACR-BHQ1-3′ TET ΔRn 5.20.1 2.9 5.6 0.8 (SEQ ID NO: 23) % of dominant allele ΔRn to total ΔRn 98%  98% 52% 86% 87% Theoretical % 100% 100% 50% 91% 91%

The above results and discussion demonstrate that 3′-ACR labeled FETnucleic acid detectors suitable for use in high fidelity PCR for allelediscrimination are provided by the subject invention. As such, thesubject methods represent a significant contribution to the art.

Example 7 Allele Discrimination Using Free MGB Under High Fidelity RealTime PCR

An 89 basepair segment of the human methylenetetrahydrofolate reductase(MTHFR) gene was amplified using primers MTHFR-F1 and MTHFR-R1 listed inTable 1. Real Time amplifications and detection were performed in ABIPRISM 7700 using 30 μl reactions that contained 20 mM Tris-HCl (pH8.3),60 mM KCl, 5.3 mM MgCl₂, 0.2 mM dATP, 0.2 mM dTTP, 0.2 mM dGTP, 0.2 mMdCTP, 0.15 uM of reference dye ROX, 0.5 uM each of primers MTHFR-F1 andMTHFR-R1, FET probes, plus or minus templates containing varies numberof MTHFR wild type or mutant or both as listed in table 7, a mix of Taqand Pwo DNA Polymerase (Roche Molecular Biochemicals) at unit ratio of 5to 1. In this test 0.2 μM each of either normal or 3′-ACR-labeled FETprobes as shown in Table 7, plus or minus 0.4 μM free netropsin ordistamycin A, were used for allele discrimination. Thermal profile was95° C. 15 sec; 50 cycles of 95° C. 15 sec, 63° C. 40 sec, 70° C. 40sec., or 95° C. 15 sec; 50 cycles of 95° C. 15 sec, 57° C. 40 sec, 70°C. 40 sec.

Similar results as Table 7 can be obtained at 63° C. anneal only wheneither free netropsin or distamycin A was included in PCR reactionmixture. There was no significant amplification when using normal PCRcondition and 63° C. anneal. Similar results as Table 7 were alsoobtained at 57° C. anneal with or without netropsin or distamycin A. Thenetropsin or distamycin A acted primarily to enhance oligo's Tm,especially of AT rich sequences, so that shorter oligos/probes can beused in real time PCR to enhance allele discrimination. Such featuresshall help primers/probes design for AT rich sequences.

The above results and discussion demonstrate that the inclusion of freenetropsin or distamycin A in PCR mixture suitable for use in a varietyof different high fidelity PCR applications are provided by the subjectinvention. As such, the subject methods represent a significantcontribution to the art.

The above results and discussion demonstrate that FET labeled nucleicacid detectors suitable for use in a variety of different high fidelityPCR applications are provide by the subject invention. As such, thesubject methods represent a significant contribution to the art.

All publications and patent applications cited in this specification areherein incorporated by reference as if each individual publication orpatent application were specifically and individually indicated to beincorporated by reference. The citation of any publication is for itsdisclosure prior to the filing date and should not be construed as anadmission that the present invention is not entitled to antedate suchpublication by virtue of prior invention.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, it is readily apparent to those of ordinary skill in theart in light of the teachings of this invention that certain changes andmodifications may be made thereto without departing from the spirit orscope of the appended claims.

1-34. (canceled)
 35. A method of monitoring of a PCR amplificationreaction, the method comprising: (a) preparing a PCR amplificationreaction mixture by combining: (i) a template nucleic acid; (ii) forwardand reverse nucleic acid primers; (iii) deoxyribonucleotides; (iv) anucleic acid polymerase having 3′→5′ exonuclease activity; (v) a FETlabeled probe, the FET labeled probe comprising a nucleic acidintercalator bonded to a FET labeled oligonucleotide, wherein thenucleic acid intercalator is covalently bonded to the 3′ end of the FETlabeled oligonucleotide; and (b) subjecting the PCR amplificationreaction mixture to PCR amplification conditions; (c) monitoring thereaction mixture for a fluorescent signal from the FET labeledoligonucleotide probe to obtain an assay result; and (d) employing theassay result to monitor the PCR amplification reaction.
 36. The methodaccording to claim 35, wherein the FET labeled oligonucleotide comprisesa 3′→5′ exonuclease resistant quencher domain.
 37. The method accordingto claim 35, wherein the PCR amplification reaction is a primerextension reaction.
 38. The method according to claim 35, wherein themethod is a real-time method of monitoring the PCR amplificationreaction.
 39. A method for screening a nucleic acid sample for thepresence of first and second nucleic acids that differ from each otherby a single nucleotide, the method comprising: (a) producing a primerextension mixture that includes: (i) the nucleic acid sample; (ii) anucleic acid polymerase having 3′→5′ exonuclease activity; (iii) firstand second FET labeled oligonucleotide probes that are complementary tosaid first and second nucleic acids, respectively, wherein each of thefirst and second FET labeled probes comprise a nucleic acid intercalatorbonded to a FET labeled oligonucleotide, wherein the nucleic acidintercalator is covalently bonded to the 3′ end of the FET labeledoligonucleotide; and (b) subjecting the primer extension mixture toprimer extension reaction conditions; (c) detecting a change in afluorescent signal, if any, from the first and second FET labeledoligonucleotide probes to obtain an assay result; and (d) employing theassay result to determine the presence or absence of the first andsecond nucleic acids in the sample.
 40. The method according to claim39, wherein the FET labeled oligonucleotide comprises a 3′45′exonuclease resistant quencher domain.
 41. The method according to claim39, wherein primer extension reaction is a PCR amplification reaction.42. The method according to claim 41, wherein the method is a real-timemethod of monitoring the PCR amplification reaction.